As evidenced by U.S. Pat. No. 5,135,700 (Williams et al.), molded products having concentric interior and exterior threads are quite common. The Williams et al. patent relates to a method and apparatus for molding a part, e.g., a faucet valve "T," having both interior and exterior threads. The interior threads are formed by a core insert, the exterior threads are formed by the mold halves and the mold "parting line" is generally perpendicular to the threads and parallel to the long axis of the valve.
U.S. Pat. No. 5,223,158 (Lizenby) relates to a nut casting apparatus. Upon a superficial analysis, such apparatus appears to involve molding both interior and exterior threads. A careful analysis of the patent confirms that only exterior threads are molded. Such molding includes placing an Acme-threaded casting screw into a mold cavity and filling the space between the mold and the screw with polymer.
Centrifugal sump pumps comprise another product having concentric interior and exterior threads. More specifically, the assignee of this invention makes and sells impeller-equipped centrifugal sump pumps having what is called an "upper volute." Such upper volute forms a major part of the impeller housing and when mated with a similarly-shaped lower volute defines the pumping chamber in which the impeller rotates.
The upper volute has a tube-like upstanding outlet port or boss with threads on the interior circumference. Such threads permit attachment of a discharge pipe.
The entire shape of an upper volute is formed by two mold halves which are clamped together prior to injecting the plastic into the cavity to form the volute. Known practice has been to make the mold halves (and the "half-cavities" in them) in such a way that the mold faces (those portions of the mold halves which abut one another when the mold is clamped) are coincident with a plane which diametrically bisects the upstanding port boss.
To put it another way, each half-cavity has a surface which forms all of the length and one-half of the circumference of the boss. The interior threads are formed by mounting a plug-like insert in the mold. When the mold halves are separated from one another, the insert is pulled out.
While generally satisfactory for making an outlet port having only interior threads, such known approach would present a few problems if used to make a sump pump outlet port having both interior and exterior threads. One such problem is that because the mold "parting line" (or perhaps more accurately, the parting plane) would be diametrical with the exterior threads (as well as with the interior threads), flashing would occur on such exterior threads. Such flashing, which would be very visibly apparent, would define a knife-like ridge running generally perpendicular to the threads and parallel to the long axis of the boss. Such flashing must be removed since it would impair (or, in an aggravated case, absolutely prohibit) attachment of a threaded pipe to the boss.
Another potential problem is that there may be some shifting of the insert during the molding process and the interior and exterior threads may not be concentric. And if that occurs, the wall thickness of the port boss is not uniform and threads are likely to be mismatched. If an insert shifts position significantly, it is not all that uncommon to find a wall thickness which is unacceptably thin and the part must be discarded.
An apparatus and method which addresses some of the problems and shortcomings of the prior art would be an important advance.